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Rail-to-rail differential input amplification stage with main and surrogate differential pairs

a technology of differential input and amplification stage, which is applied in the direction of instruments, electric/magnetic computing, analogue computers, etc., can solve the problems of internal bias point changes, degradation of input pair performance, and consequently the entire amplifier

Inactive Publication Date: 2006-03-09
UT BATTELLE LLC
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Benefits of technology

[0020] In a first preferred embodiment, the invention is an essentially constant transconductance, constant slew rate, noncomplementary rail-to-rail differential input amplification stage having a common-mode input voltage range including low, mid, and high range. It uses a main input differential pair of transistors receiving input signal voltages from an external source. It employs a surrogate input differential pair of transistors biased with a voltage offset from the main input differential pair of transistors. It also includes a means for controllably isolating the surrogate input differential pair of transistors from the main signal path, a means for sensing the amount of bias current in the main input differential pair of transistors, and a means for providing a controlled amount of bias current to the surrogate input differential pair of transistors.
[0021] In another preferred embodimen

Problems solved by technology

This is because when the common-mode input voltages are near the supply rails, they cause changes in internal bias points.
This, in turn, can lead to degradation in performance of the input pair and consequently the entire amplifier.

Method used

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  • Rail-to-rail differential input amplification stage with main and surrogate differential pairs
  • Rail-to-rail differential input amplification stage with main and surrogate differential pairs
  • Rail-to-rail differential input amplification stage with main and surrogate differential pairs

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Embodiment Construction

Noncomplementary Rail-to-Rail Differential Input Amplification Stage

[0024] A preferred embodiment of the invention is the noncomplementary rail-to-rail differential input amplification stage shown in FIG. 1. An example using n-channel CMOS devices is described. Because there is only one polarity of transistor in the main amplification path (M1a, M2a, M3a, M4a, M8a, and M9a), we call this a noncomplementary topology. In FIG. 1, Vin1 and Vin2 are the input signal voltages to the stage that come from an independent external source. Points A and B are output terminals that connect to the differential inputs of a subsequent stage.

[0025] In FIG. 1, M1a and M2a comprise a standard differential input pair of transistors (main transistors) biased with a current flowing through M10a set by VcurrentA. M3a and M4a are differential surrogate transistors, and M8a and M9a are cascode isolation transistors whose gates are biased with the voltage Vbias1. The cascode isolation transistors M8a and M...

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Abstract

An operational amplifier input stage provides a symmetrical rail-to-rail input common-mode voltage without turning off either pair of complementary differential input transistors. Secondary, or surrogate, transistor pairs assume the function of the complementary differential transistors. The circuit also maintains essentially constant transconductance, constant slew rate, and constant signal-path supply current as it provides rail-to-rail operation.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0001] The United States Government has rights in this invention pursuant to Contract No. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC.BACKGROUND OF THE INVENTION [0002] 1. Field of Invention [0003] The invention relates to operational amplifier input stages having essentially constant transconductance and slew rate. More particularly, it relates to an input stage that incorporates a secondary, or surrogate, differential pair of transistors that remain off during normal midrange operation but gradually replace the main differential pair of transistors when the common-mode voltage approaches the supply rails. [0004] 2. Description of Prior Art [0005] Operational amplifiers used in low-voltage CMOS integrated-circuit processes normally require a differential input stage that can operate properly even when the common-mode voltages are near the supply rails. This is because when the ...

Claims

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Application Information

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IPC IPC(8): H03F3/45
CPCH03F3/45183
Inventor BRITTON, CHARLES LANIER JR.SMITH, STEPHEN FULTON
Owner UT BATTELLE LLC